1. Field of the Invention
The present invention relates to an equalization technique. More particularly, the present invention relates to an equalization technique for a channel with multiple clusters.
2. Description of Related Art
In a wireless communication environment, there may be a multi-path phenomenon due to diffractions and refractions of electromagnetic waves caused by obstacles located between a receiver and a transmitter. Therefore, when a channel thereof is observed in time domain, the channel may have a plurality of delay paths, while when the channel is observed in frequency domain, the channel may be regarded as a frequency-selective channel. As to the present various communication systems, a spread-spectrum system is more likely to be interfered by the frequency-selective channel, which may decrease a performance of the receiver.
Taking a present code division multiple access (CDMA) system for an example, to solve the problem of an interference in a frequency-selective channel, a receiver of the CDMA system generally applies an equalization technique for equalizing the frequency-selective channel. In other words, an equalizer is used to convert the frequency-selective channel into a frequency-flat channel, so as to reduce the multipath interference.
FIG. 1 is a system block diagram of a conventional CDMA receiver applying an equalizer. Referring to FIG. 1, a channel response of a received signal r[m] is estimated by a channel estimation unit 110, namely, a delay time τ of each delay path within the channel and a channel gain ĥ(τ) corresponding to each delay time are estimated, and a plurality of weights w0, w1, w2, . . . , wF−1 of an equalizer 130 are estimated according to the estimated channel gains ĥ(τ), and then the weights w0, w1, w2, . . . , wF−1 are output to the equalizer 130. Next, the equalizer 130 sequentially delays the received signals r[m] for a chip duration TC. Then, after respectively multiplying the original received signal r[m] and the delayed received signals r[m−1], r[m−2], . . . , r[m−F+1] with the weights w0, w1, w2, wF−1, a combination of above multiple multiplications is then output. A correlator 150 de-spreads the equalized signal according to a spreading code c[n], and a decision unit 170 is used for demodulating a digital signal {circumflex over (b)}.
A window length of the equalizer 130 is represented by F. For a present equalization technique, a plurality of documents (for example, note [1]) refers to that the window length F of the equalizer has to be greater than or equal to twice of a channel length thereof, so that the equalizer may effectively eliminate the interference of the channel to the received signals. Therefore, from the viewpoint of hardware implementation in a receiver, if the window length of channel estimation is L, the window length F of the equalizer is then designed to be 2L.
However, in a channel with relatively large delay spread, the multipath signals might be separated quite far apart, as shown in FIG. 2. FIG. 2 is a diagram illustrating a channel power delay profile. Referring to FIG. 2, horizontal coordinates thereof represent delay times τ with a unit of nanosecond (ns), and vertical coordinates represent powers |ĥ(τ)|2 with a unit of dB. According to FIG. 2, the delay paths within the channel are sparsely distributed in time domain, and the delay paths may be grouped into two clusters of cluster 1 and cluster 2. The cause of such channel phenomenon may be that in a hilly terrain (HT), the electromagnetic waves emitted from the transmitter are received by the receiver after a long distance reflection, so that the delay paths of the cluster 2 are generated. Alternatively, the receiver may be just located within transmission ranges of two base stations, so that the receiver may simultaneously receive signals from the two base stations, and therefore the delay paths of the cluster 1 and the cluster 2 are generated.
Consider a channel environment of FIG. 2, due to a limitation of the hardware, if the window length of the equalizer of the receiver maintains to be F=2L, here L is assumed to be the channel length of cluster 1, the window length of the equalizer is not enough for each of the delay paths within the channel, so that the equalizer cannot equalize the transmission channel, and accordingly performance of the receiver is degraded.
A U.S. patent laid-open publication No. 2006/0109892 A1 provides a receiver having two equalizers, as shown in FIG. 3. Wherein, the two equalizers 335 and 340 of the receiver 300 equalize the received signals respectively based on delay paths 305A and 305B of two clusters. Next, the signals equalized by the two equalizers 335 and 340 are combined for outputting to a CMIS circuit 352. The CMIS circuit 352 reconstructs the interfering signals, which are used to be subtracted from the received signal through the adders 325 and 330.
According to the aforementioned U.S. patent laid-open publication, when the weights are calculated, calculation of the weights of equalizer 335 only considers a channel response of the delay path 305A of the first cluster, and calculation of the weights of equalizer 340 only considers a channel response of the delay path 305B of the second cluster. In other words, the weights of the equalizers 335 and 340 are not calculated under a minimum mean square error (MMSE) criterion. Actually, in such a two-cluster channel, the signal received by the equalizer 335 is interfered by the delay path 305B of the second cluster. However, the equalizer 335 may only mitigate the interference of the delay path 305A of the first cluster. Similarly, the signal received by the equalizer 340 is interfered by the delay path 305A of the first cluster. But the equalizer 340 may only mitigate the interference of the delay path 305B of the second cluster. Therefore, though the two equalizers 335 and 340 are applied in the aforementioned patent, interferences of the delay paths 305A and 305B cannot be simultaneously mitigated. Since the equalizers 334 and 340 cannot totally eliminate the interferences within the channel, the signal reconstructed by the CMIS circuit still may contain interference. While the reconstructed signal polluted by interference is still fed back to the adders 325 and 330, the error propagation in the receiver occurs. Moreover, when the signal-to-interference plus noise ratio (SINR) at the receiver is low, such feed-back mechanism may lead to the error propagation, and hence degrade the reception performance.    Note [1]: M. Melvasalo, P. Jänis and V. Koivunen., “Low complexity space-time MMSE equalization in WCDMA systems,” proc. of 2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications, Berlin, Germany, pp. 306-310, 2005.